Re: [PATCH] mm: Prevent racy access to tlb_flush_pending

[Andy Lutomirski <luto@kernel.org>]

On Mon, Jul 17, 2017 at 11:02 AM, Nadav Amit <namit@vmware.com> wrote:
> Setting and clearing mm->tlb_flush_pending can be performed by multiple
> threads, since mmap_sem may only be acquired for read in task_numa_work.
> If this happens, tlb_flush_pending may be cleared while one of the
> threads still changes PTEs and batches TLB flushes.
>
> As a result, TLB flushes can be skipped because the indication of
> pending TLB flushes is lost, for instance due to race between
> migration and change_protection_range (just as in the scenario that
> caused the introduction of tlb_flush_pending).
>
> The feasibility of such a scenario was confirmed by adding assertion to
> check tlb_flush_pending is not set by two threads, adding artificial
> latency in change_protection_range() and using sysctl to reduce
> kernel.numa_balancing_scan_delay_ms.

This thing is logically a refcount.  Should it be refcount_t?

--Andy

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